It is known that pure gold is extremely soft while in the production of jewelry products there is required a material which offers particular properties of hardness, resistance to wear and at the same time which maintains a warm and brilliant color.
The hardness of an alloy based on gold depends both on phenomena of precipitation in the solid phase and also on phenomena of transition of disorder/order which occur below certain temperatures and for determined atomic ratios between the elements of the alloy, a property which in particular is achieved during the thermal hardening treatment. It is actually during this thermal treatment that titanium becomes a determining factor, because while it participates to the phenomena of precipitation mentioned hereinabove, it also provides new properties to the alloy of which is a component.
The alloys based on gold in general are made starting from the ternary system Au--Ag--Cu and commercially they are available with different titers of gold varying between 8 to 24 Karats. There are also commercially available intermediate amounts of alloys, specifically with 9, 10, 12, 14, 18, 20, 21 and 22 Karats.
Up to very recent times the study of gold, a metal used for centuries for ornamental purposes, particularly for its color and its resistance to staining, has been limited to alloys which are easily workable and which have a brilliant color. With the increase in the price of gold the necessity of using jewelry lighter in weight but also resistant has become very significant. In addition, in the case of more showy articles, particularly hollow bracelets, earrings in the form of a circle and very recently the production of very thin chains similar for instance to the products listed in the Italian application number 95A000085 in the name of the same applicant as the present application, there are required products which are lighter in weight and resistant for reasons not only economical but also aesthetic in appearance.
This requirement has necessitated that other metals in the pure state be added to the three elements Au--Ag--Cu.
The metal which to a greater extent is present to form the quaternary alloy on the base of gold is zinc which is added in small amounts, less than 0.5% as a deoxidizing agent or in greater amounts up to 10% or even more when it is necessary to modify the color, the work-ability and the mechanical properties. The system of alloys of gold, silver, copper and zinc (Au--Ag--Cu--Zn) comprises almost all the gold alloys commonly used for the production of jewelry articles. These alloys may be different one from the other due to the addition to the four elements of small percentages of other metals such as nickel (Ni), cobalt (Co), iron (Fe), silicon (Si), boron (B), ruthenium (Ru), iridium (Ir), indium (In) etc. By way of example patents which demonstrate what has been mentioned hereinabove and which are characterized by the fact that they offer a variety of components which contribute to form an alloy based on gold are:
U.S. Pat. No. 2,141,157 in the name of Peterson contains 33% up to 84% of gold, 10% up to 67% of copper, between 2% up to 10% of zinc, between 2% up to 10% of silver and between 0.1% up to 5% of cobalt. In this alloy cobalt is used to obtain a reversible hardening.
U.S. Pat. No. 2,229,463 in the name of Leach contains 35% up to 75% of gold, between 5% up to 25% of silver, between 12% up to 35% of copper, between 0.1% up to 12% of zinc and between 1% up to 5% of iron. In this alloy iron is used to obtain both a reversible and irreversible hardening.
U.S. Pat. No. 2,248,100 in the name of Loebich contains between 33% up to 66% of gold, between 1% up to 30% of silver, between 10% up to 55% of copper, between 0.5% up to 15% of zinc and between 0.1% up to 5% of iron.
U.S. Pat. No. 3,981,723 describes an alloy of gold which contains palladium, silver, indium and 0.005% of iridium or ruthenium.
More recently the alloys described in U.S. Pat. No. 5,180,551 in the name of Leach & Garner and U.S. Pat. No. 5,173,132 in the name of Solomon utilize different percentages of combinations of cobalt and nickel for the purpose of improving substantially the grain structure, to improve the color and increasing the mechanical systems.
As shown in the above list of patents the titanium (Ti) element has not been used because the working of alloys containing titanium requires particular measures due to the substantial reactivity at the high temperatures with air, oxygen, nitrogen and carbon, the nitrides and carbides which are commonly present in forming the protective atmosphere during the treatments of solubilization, of aging, cold working, fusion and others.
As an element of gold alloy titanium is present at the present state of the art in the production of unfinished products used in the industry, in particular in the industry of electronic components such as Japanese J5 9107-043-A, Russian RU 2012605-C1, German publication DD-145-183 or for the production of the gold alloy usually called "white gold" as described in Japanese publication JO 3130-334-A which offers, however, physical properties substantially different from the usual ternary and quaternary gold alloys.
As an element of gold alloy used in jewelry, titanium is present according to the present state of the art only in the well known gold alloy called "gold 990". The gold alloy 990 is an alloy having high Karats which originated as a result of a study in the 1970 period and which was intended to solve two problems, hardening an alloy with a very low percentage of alloying element and at the same time producing an alloy which satisfied aesthetic criteria such as color. The result has been an alloy with 23.76 Karats of gold and 1% of titanium by weight which offered a resistance to wear compared with an 18 Karats alloy claimed in particular in U.S. Pat. No. 1,023,334 and patent P35.02 914.5. Titanium in addition is present in the form of titanium nitride in the treatment of superficial hardening to increase resistance to wear and abrasion of jewelry articles having high Karats (see A. R. Zielonka Gold Technology, November 1994).